1. Field of the Invention
The present invention relates to an opposed impeller arrangement; and more particularly relates to a pump having such an opposed impeller arrangement.
2. Brief Description of Related Art
By way of example, FIG. 1 shows part of a conventional multi-stage opposed impeller i (see FIG. 4) that is known in the art and includes a shaft labelled as 1, a stage 1 impeller labelled as 3 and a stage 2 impeller labelled as 4. FIG. 1 also shows the stage 1 wear ring diameter labelled as 2 and the stage 2 wear ring diameter labelled as 5. (In FIG. 1, all five of these reference labels appear in a circle). FIG. 1 shows the suction pressure P1 into the stage 1 inlet, the stage 1 discharge pressure P2 and the stage 2 discharge pressure P3; pressure percentages (e.g., indicated by arrows and pressure indicators P1, P2, % P2, % P3), e.g., between P1 and % P2 for stage 1 and between for P2 and % P3 for stage 2; and a pressure differential indicated by an arrow labeled P3−P2.
FIG. 4 shows the conventional multi-stage opposed impeller i having the stage 1 wear ring, the stage 2 wear ring, the impeller stage 1, the impeller stage 2, e.g., arranged on the pump shaft.
In the prior art, and consistent with that shown in FIGS. 1 and 4, a normal multi-stage opposed impeller pump utilizes two or more impellers that may or may not be of identical design and construction with the inlets in opposite directions. In some cases the second stage inlet may have a different size than the first stage inlet. These inlets are called the eye of the impeller. If the impellers are of the same design and construction it helps to reduce radial and axial forces generated within the pump and through the operating range. However, some design may make the stage 1 inlet to be designed for improved suction performance and as a result may have a larger impeller eye diameter. The second stage eye may be smaller due to the fact that it is receiving pressure from the first stage discharge which helps to prevent cavitation, and to improve overall pump efficiency.
Some of the shortcomings of the above mentioned devices include the following: Having 2 identical impellers helps to reduce the axial forces generated but typically there is still an imbalance due to the higher pressure located at the inlet of the second stage. When the impellers have a different size inlet, this allows for an even greater imbalance in axial forces, but can also lead to a difference in design of the second stage wear ring, additional components, and complexity. If the same wear ring is not used, a second wear ring will need to be used which now increases the axial imbalance and may make the machining of the pump casing more complex.
There is a need in the industry for a better way to configure these known impellers presently used in multi-stage opposed impeller pumps.